In a conventional method conducting a closed-loop power control for a reverse link, a signal power from a mobile station (MS) is estimated at a base station (BTS), the estimated power is then compared with critical power magnitude required for maintaining signal quality. According to the comparison result, the BTS transmits a TPC (Transmit Power Control) bit for commanding the MS to increase or decrease current transmitting power level of a MS. Receiving the TPC bit, the MS interprets it and increments or decrements the transmitting power level stepwise. The power level adjusting resolution is 1.0 dB.
FIG. 1 illustrates a functional block diagram of a power controlling unit installed in a mobile station.
A MS receives downlink (forward link) signals from a neighboring BTS, then, an automatic gain controller (AGC) 11 adjusts its gain to flatten the received signal level, a demodulator 12 extracts the TPC bit from the received downlink signals, a TPC interpreter 13 interprets which operation the extracted TPC bit is requesting. A power level controller 15 determines whether to increment or decrement closed-loop power level by the adjusting step Pclosed based on the interpretation, and adds the determined ±Pclosed to an open-loop power control level ΔPopen, which is determined by a RSSI 14 based on the level of the output signal of the AGC 11. A power adjusting signal for the total power control level ΔPt (=ΔPopen±Pclosed) is applied to a high-power amplifier (HPA) 16 from the power level controller 15 so that the current power level of uplink signals is adjusted by ΔPt.
The power level of signals transmitted from a MS is estimated every 1.25 msec at a BTS. The time 1.25 msec is equal to duration of 6 Walsh symbols and is called a power control group (PCG). Therefore, sixteen power control groups are contained in a 20 msec-long traffic frame.
A BTS transmits 1-bit TPC command to a MS based on the estimated power level every PCG. Thus, the power level controller 15 of a MS outputs the 1 dB power increment signal to the HPA 16 if the value interpreted every 1.25 msec is ‘1’, and it outputs 1 dB power decrement signal if ‘0’.
However, the 1-bit TPC information is frequently distorted due to wireless environment, and if a receiving power level changes very rapidly or slowly (a power level changing speed is mainly affected by the moving speed of a MS), it is difficult to track the variation of the power level through the only 1 dB increment or decrement.
For example, supposed that power level received at a BTS is the graph of PRx as shown in FIG. 2, it is ideal that the transmitting signal power level controlled by the power level controller 15 is the graph of PTx. However, 1 dB step (ΔP) adjustment conducted every 1.25 msec can not track the graph PTx exactly as shown in FIG. 2 when the variation of power level to compensate is too sharp since the moving speed of a MS is very high. In addition, when the variation of power level is too small, 1 dB-step adjustment may cause an oscillation of transmitting power level.